THE ORIGIN AND EVOLUTION OF THE VERTEBRATES 73 



bottom. The secretion of a protective coat or tunic would be a 

 logical consequence of such a step and we would have the typical 

 tunicates. The salpians probably were an offshoot of the primitive 

 tunicates that migrated away from the shores, but instead of 

 following the bottom they acquired the floating or pelagic habit. 



A totally different situation, however, would meet those primitive 

 lancelets that migrated from the shores up the mouths of rivers. 

 The water currents would become more constant and there would be 

 less opportunity for sedentary life. In river currents the microscopic 

 organic particles suspended in the water would be much less abundant 

 and hence it would be more difficult to subsist by the old lancelet 

 method of food concentration. This whole apparatus as a feeding 

 apparatus would therefore lose its significance and would be used 

 largely as a respiratory mechanism. Perhaps the first part of the 

 mechanism to be lost would be the one last formed in ontogeny — 'the 

 atriiun. Possibly the remains of the atrium would persist as paired 

 lateral ridges, or metapleural folds, that might act as lateral flanges 

 or balancing organs in swimming, and may have been the primordia 

 of the paired fins of fishes. New feeding methods had to be acquired 

 and at least two schemes were adopted, one the jaw apparatus involv- 

 ing the opening of a new ventral mouth, and the second the oral fun- 

 nel apparatus with its accessory, the rasping tongue. The jaw appa- 

 ratus characterizes the true fishes (Pisces), while the oral funnel and 

 rasping "tongue" apparatus characterizes the Cyclostomata. Both 

 of these groups appear to have originated at about the same time and 

 independently of each other. In many ways the cyclostomes have 

 been much the less successful type and comparatively little evolution- 

 ary progress has resulted. The true fishes, on the other hand, appear 

 to have furnished the basis of all future vertebrate specialization. 

 That the rapid stream environment was the stimulating factor in 

 the production of the first true vertebrates has been ably upheld by 

 Prof. T. C. Chamberlin, on purely theoretic grounds. He contends 

 that the sea does not furnish the dynamic stimuli necessary to bring 

 about the evolution of vertebrate characters. The rivers, however, 

 furnish just the needed conditions to bring forth the complex of energy 

 elements that we associate with a vertebrate. 



" There is only one conspicuous type that is facilely suited to 

 free life, independent of the bottom, in swift streams, and that 

 is the fish-form. The form and the motion 6f the typical fish are 



